The find and getReference methods are not required to be invoked within a transaction context. If an entity manager with transaction-scoped persistence context is in use, the resulting entities will be detached; if an entity manager with an extended persistence context is used, they will be managed. See section 3.3 for entity manager use outside a transaction

According to the spec my explanation is correct.

in other words only managed entities can be removed. that means to use remove method you need a managed entity not just id. so there is no remove(Id) method